Ibrahimovic is out until January 2018, as he continues to recover from a knee injury.
Lukaku has hit the ground running at Old Trafford, scoring 10 goals in his first nine United matches, since his £75million move from Everton last summer.
Mourinho admits he cannot rotate the 24-year-old yet.
“Without Zlatan we can’t rotate the strikers.
“Also because Marcus Rashford is playing in other postions.
“Until the moment we have Zlatan we can’t think about giving rest to our number nine the same way we give in other positions. We can’t do that,” the Portuguese said.